Such network terminations are needed at the subscriber interface in various digital communications networks. For example, there are standard network terminations and primary multiplex network terminations for ISDN (2 Mb). Network terminations are also used in ATM (asynchronous transfer mode)-based digital communications networks and in networks based on other transmission techniques.
An article by W. Harderich ("ISDN-Produkte fur Europa", telekom praxis 3/94, pp. 29-35) describes that a network termination for ISDN is made available to a subscriber and installed by a network operator. On the other hand, however, the network termination is on the subscriber's premises and is supplied with power by the subscriber.
A problem associated with this constellation arises if the power supply of the network termination is interrupted by a power failure at the subscriber. The subscriber will notice at a terminal only that a malfunction has occurred in the communications network but cannot locate the error and will not realize that the correction of the error falls within his or her purview.
The same problem is encountered with leased lines by which a customer interconnects different ones of his subnetworks via a public network. Here, too, network terminations of the public network operator are installed on the customer's premises and supplied with power from the customer, and it is not possible for the customer to detect a power failure at the network terminations. For 2-Mb PCM (pulse code modulation) connections, an error indication bit (SA bit) is defined in the frame of the message signals which is set in the event of a power failure at the network terminations in order to signal the error, but this error indication bit can be transmitted only to the public network operator, not to the customer.